@BAYRAM_GUVEN
Best Advice would be stay out of the Level BP - I know that sounds strange but it’s hard to access from & to the Level BP so for anyone starting or anyone basically it would be advised just to stay away from it for the most part.
an exception would be something like a door BP that opens on it’s own when something enters the trigger box which you can do an Event (lower right side in a BP to select an Event such as Begin Overlap etc) then cast to see if it’s the Pawn or Player Character and/or some condition has been met but still such a BP really isn’t communicating with the actual Level BP it just exists in it.
to keep from being too confusing try watching this Video: https://www.youtube.com/watch?v=EM_HYqQdToE
( I know it was done in 2015 but it covers a lot of basics that all should work )( it should help you a lot)
Good Luck with your Journey in Unreal !! 